The present invention relates to a method for forming metal films as part of metallization processing of particular utility in integrated circuit semiconductor device and circuit board manufacture, and is especially adapted for use in processing employing "damascene" (or "in-laid") technology.
The escalating requirements for high density and performance associated with ultra large scale integration (ULSI) semiconductor device wiring are difficult to satisfy in terms of providing submicron-sized (e.g., 0.15 .mu.m), low RC time constant metallization patterns, particularly wherein the submicron-sized metallization features such as vias, contact areas, grooves, trenches, etc., have high aspect (i.e., depth-to-width) ratios due to microminiaturization.
Semiconductor devices of the type contemplated herein typically comprise a semiconductor wafer substrate, usually of doped monocrystalline silicon, and a plurality of sequentially formed interlayer dielectrics and conductive patterns formed therein and/or therebetween. An integrated circuit is formed therefrom containing a plurality of patterns of conductive lines separated by interwiring spacings, and a plurality of interconnect lines, such as bus lines, bit lines, word lines, and logic interconnect lines. Typically, the conductive patterns of vertically spaced apart metallization layers are electrically connected by a vertically oriented conductive plug filling a via hole formed in the dielectric layer separating the layers, while another conductive plug filling a contact hole establishes electrical contact with an active region, such as a source/drain region, formed in or on the semiconductor substrate. Conductive lines formed in groove or trench-like openings in overlying dielectric layers extend substantially parallel to the semiconductor substrate. Semiconductor devices of such type fabricated according to current technology may comprise five or more levels of such metallization in order to satisfy device geometry and miniaturization requirements.
Electrically conductive films or layers of the type contemplated herein for use in e.g., "back-end" semiconductor manufacturing technology as required for fabrication of devices as above described typically comprise a metal such as titanium, tantalum, tungsten, aluminum, chromium, nickel, cobalt, silver, gold, copper, and their alloys. In use, each of the recited metals presents advantages as well as drawbacks. For example, aluminum (Al) is relatively inexpensive, exhibits low resistivity, and is relatively easy to etch. However, in addition to being difficult to deposit by lower cost, lower temperature, more rapid "wet" technology such as electrodeposition, step coverage with aluminum is poor when the metallization features are scaled down to submicron size, resulting in decreased reliability of interconnections, high current densities at certain locations, and increased electromigration. In addition, low dielectric constant materials, e.g., polyamides, when employed as dielectric interlayers, create moisture/bias reliability problems when in contact with aluminum.
The use of via plugs filled with tungsten (W) may alleviate several problems associated with aluminum. However, most W-based processes are complex and expensive. In addition, the high resistivity of W may cause Joule heating which can undesirably enhance electromigration of aluminum in adjacent wiring. Moreover, W plugs are susceptible to void formation and high contact resistance at the interface with the aluminum wiring layer.
Copper (Cu) and Cu-based alloys are particularly attractive for use in large-scale integration (LSI), very large-scale integration (VLSI), and ultra large-scale integration (ULSI) devices requiring multilevel metallization systems for "back-end" processing of the semiconductor wafers on which the devices are based. Cu and Cu-based metallization systems have very low resistivities, i.e., significantly lower than that of W and even lower than those of previously preferred systems utilizing Al and its alloys, as well as significantly higher resistance to electromigration. Moreover, Cu and its alloys enjoy a considerable cost advantage over a number of the above-enumerated metals, notably silver and gold. Also, in contrast to Al and the refractory-type metals, Cu and its alloys can be readily deposited at low temperatures in good quality, bright layer form by well-known electroplating techniques, at deposition rates fully compatible with the requirements of device manufacturing throughput.
In addition to convenient, relatively low cost, low temperature, high throughput "wet" deposition by electroplating, Cu and its alloys are readily amenable to low cost, high throughput electroless deposition of high quality films for efficiently filling recesses such as vias, contact areas, and grooves and trenches forming interconnection routing. Such electroless plating generally involves the controlled autocatalytic deposition of a continuous film of Cu or an alloy thereof on a catalytic surface by the interaction of at least a Cu salt and a chemical reducing agent contained in a suitable solution, whereas electroplating comprises employing electrons supplied to an electrode from an external source (i.e., a power supply) for reducing Cu ions in solution and depositing reduced metal atoms on the surface thereof. In either case, a nucleation/seed layer is required for catalysis and/or deposition on the types of substrates contemplated for use herein. Finally, while electroplating requires a continuous nucleation/seed layer, very thin and discontinuous islands of a catalytic metal may be employed with electroless plating.
As indicated above, a commonly employed method for forming "in-laid" metallization patterns such as are required for "back-end" metallization processing of semiconductor wafers employs "damascene" type technology. Generally, in such processing methodology, a recess (i.e., an opening) for forming, e.g., a via hole in a dielectric interlayer for electrically connecting vertically separated metallization layers, is created in the dielectric interlayer by conventional photolithographic and etching techniques, and filled with a metal plug, typically of W. Any excess conductive material (i.e., W) on the surface of the dielectric interlayer is then removed by, e.g., chemical-mechanical polishing techniques (CMP), wherein a moving pad is biased against the surface to be polished, with the interposition of a slurry containing abrasive particles (and other ingredients) therebetween.
A variant of the above-described technique, termed "dual damascene" processing, involves the formation of an opening comprising a lower contact or via hole section in communication with an upper groove or trench section, which opening is filled with a conductive material, typically a metal, to simultaneously form a conductive plug in electrical contact with a conductive line.
Referring now to FIG. 1, schematically shown therein in simplified cross-sectional view is a conventional damascene processing sequence employing relatively low cost, high manufacturing throughput electroplating and CMP techniques for forming recessed "back-end" metallization patterns (illustratively of Cu-based metallurgy but not limited thereto) in a semiconductor device formed in or on a semiconductor wafer substrate 1. In a first step, the desired arrangement of conductors is defined as a pattern of recesses 2 such as via holes, grooves, trenches, etc., formed (as by conventional photolithographic and etching techniques) in the surface 4 of a dielectric layer 3 (e.g., a silicon oxide and/or nitride or an organic polymeric material) deposited or otherwise formed over the semiconductor substrate 1. In a second step, a layer of Cu or Cu-based alloy 5 is deposited by conventional electroplating techniques to fill the recesses 2. In order to ensure complete filling of the recesses, the copper-containing layer is deposited as a blanket (or "overburden") layer of excess thickness t so as to overfill the recesses 2 and cover the upper surface 4 of the dielectric layer 3. Next, the entire excess thickness t of the metal overburden layer 5 over the surface 4 of the dielectric layer 3 is removed by CMP utilizing an alumina-based slurry, leaving metal portions 5' in the recesses 2 with their exposed upper surfaces 6 substantially coplanar with the surface 4 of the dielectric layer 3.
The above-described conventional damascene process forms in-laid conductors 5' in the dielectric layer 3 while avoiding problems associated with other types of metallization patterning processing, e.g., blanket metal layer deposition, followed by photolithographic masking/etching and dielectric gap filling. In addition, such single or dual damascene-type processing can be performed with a variety of other types of substrates, e.g., printed circuit boards, with and/or without intervening dielectric layers, and with a plurality of metallization levels, i.e., five or more levels.
However, the use of electroplated metallization as described above has presented a number of problems, particularly, but not exclusively, with the use of Cu-based metallurgy. For example, although electroplating of Cu (a "wet" technique) has advantages over "dry" techniques (e.g., physical or chemical vapor deposition), such as rapid rates of deposition at low temperatures and good compatibility with "wet" CMP processing, it suffers from a drawback of ridge build-up over sharp corners of vias, grooves, and trenches. Thus, in conventional practices utilizing electrolytic deposition of Cu or Cu-based conductors, a rather thick blanket or overburden layer 5, typically about 0.5-1.5 .mu.m thick, must be deposited over the recess-patterned surface to ensure complete filling (i.e., overfilling) of recesses 2 such as via holes, trenches, grooves, and other variously configured openings. Moreover, the resulting surface after overfilling may be highly non-planar, with the layer thicknesses thereof spanning the entire range of thicknesses given above.
Removal of such thick, non-planar blanket layers of Cu-based material in the subsequent CMP step for planarizing the interconnection metallization entails a number of disadvantages. For example, removal of the excess Cu-based material by CMP is slow and expensive. Specifically, typical Cu or Cu alloy removal rates by CMP employing a conventional alumina-based slurry are on the order of about 2,000-3,000 .ANG./min. Consequently, removal of 0.5-1.5 .mu.m thick copper-based layers can require long processing times extending up to about 5 minutes, considerably longer than that desired for good manufacturing throughput and reduced expense. In addition, removal of such thick as-deposited Cu or Cu-based blanket or overburden layers by CMP results in less uniform polished layers as are obtained when CMP is performed on thinner deposited layers. Such poor uniformity is generally accompanied by an increase in defects such as non-planarity ("dishing") and gouging ("erosion") between adjacent metallization lines.
A further drawback associated with Cu-based "back-end" metallization is Cu diffusion into the underlying semiconductor, typically silicon, resulting in degradation of the semiconductive properties thereof, as well as poor adhesion of the deposited Cu or Cu alloy layer to various materials employed as dielectric interlayers, etc. As a consequence of these phenomena associated with Cu-based metallurgy, it is generally necessary to provide an adhesion promoting and/or diffusion barrier layer intermediate the semiconductor substrate and the overlying Cu-based metallization layer. Suitable materials for such adhesion/barrier layers include, e.g., titanium, tungsten, chromium, tantalum, and tantalum nitride.
Yet another drawback associated with the use of electroplated Cu or Cu-based damascene type metallization arises from incomplete filling of the recesses during the electroplating process, resulting in void and/or other defect formation causing a reduction in device quality. Referring now to FIGS. 2A-2B and 3A-3B, wherein like reference numerals are employed as previously to designate like features, illustrated therein are simplified schematic cross-sectional views showing sequential phases of the filling of a damascene type recess 2 formed in a dielectric layer 3 overlying a semiconductor wafer substrate 1, typically monocrystalline silicon, with an electroplated metal layer 5, e.g., of Cu or an alloy thereof. Referring more particularly to FIG. 2A, as illustrated therein, a thin adhesion/barrier layer 7 of chromium, tantalum, or tantalum nitride and an overlying thin nucleation/seed layer 8 of refractory metal, Cu, or Cu-based alloy have been sequentially deposited in conventional thicknesses (by conventional techniques such as PVD, CVD, and PECVD) over the surfaces of the dielectric layer exposed within recess 2 and the non-recessed surface 4.
Referring now to FIG. 2B, shown therein is a typical "after-plating" view of recess 2, illustrating formation of an unfilled region 9 in the Cu or Cu alloy plug portion 5' of metallization layer 5, which unfilled region constitutes an undesirable void or defect resulting in lowered device quality and performance characteristics. While the exact mechanism of such occlusion or "pinching-off" of recess 2 at the upper, or mouth portion 2', thereof is not known with certainty, it is believed to result from increased rates of copper electroplating at the corners of the nucleation/seed layer 8, e.g., at portions indicated by reference numeral 8A. It is further believed that such increased rates of deposition at corners 8A are related to the formation of higher electric fields at such corners during application of the electrical potentials necessary for effecting electroplating thereon.
The problem of recess occlusion or "pinching-off" during filling by Cu electroplating is further exacerbated, when, as is illustrated in FIGS. 3A-3B analogous to FIGS. 2A-2B, the corners of the nucleation/seed layer 8 include overhanging portions 8A which are frequently formed as a result of conventional processing techniques (PVD, CVD, etc.) for forming same. Such overhang formation further constricts the opening dimension of the recess 2 at the mouth portion 2' thereof and consequently increases the likelihood of "pinch-off" and concomitant void 9 formation.
As design rules for LSI, VLSI, and ULSI semiconductor devices extend further into the submicron range, e.g., about 0.18 .mu.m and below, and the number of metallization levels increases, the reliability of the metallization/interconnect pattern becomes increasingly critical. Accordingly, the problem of "pinching-off" or occlusion of recess openings during metal (e.g., Cu or Cu-based) filling thereof by electroplating requires amelioration.
Thus there exists a need for metallization process methodology enabling the formation of metal contact and interconnect members, particularly of Cu or Cu-based alloys, having high reliability, high yield, and high performance. In particular, there exists a need for eliminating the problem of void formation in metal recess-filling plugs which results when recesses are filled by conventional electroplating techniques.
The present invention addresses and solves the problems attendant upon conventional processes for manufacturing semiconductor devices utilizing electroplated Cu metallization, particularly in the formation of in-laid "back-end" contacts/metallization patterns by damascene techniques employing electroplating and CMP for obtaining good manufacturing throughput and product quality.